E.coli aspartate transcarbamoylase is one of the most characterized allosteric proteins. Solution scattering curves have been effectively used to study quaternary structure of the enzyme up on binding substrates and substrate analogs. We recorded solution scattering curves from a couple of mutant versions of the enzyme in order to pinpoint the role of 80's loop in the catalytic chain of this dodecameric enzyme. The amino acid residues in the 80's loop undergo substantial changes in salt bridges. We have shown that there exists a quaternary structure which is very similar to fully activated R-state but does not form a high-affinity complex with substrates, thus possessing very low catalytic activity.